(Optional) Method in Checkpoint actor clear past checkpoints

[aakoshh]

At some point, if we see that the checkpoint has been registered in the parent, we may want to clear out past checkpoints to free up space in our ledger.

[dnkolegov]

@aakoshh Should we also have a variable H in the actor state, which means all checkpoints with height <= H have been committed and may be removed? And no one else can add a signature for that checkpoint.

What functions Fendremint does need?

removeCheckpoints(uint256 lastCheckpointHeight) - remove all checkpoints with height h <= lastCheckpointHeight
removeCheckpoint(uint256 checkpointHeight) - removethe checkpoints with height checkpointHeight
removeCheckpoints(uint256[] checkpointHeights) - removethe checkpoints with heights from checkpointHeights

[dnkolegov]

Implemented - consensus-shipyard/ipc-solidity-actors#207